create built-in substitutes for fireflies and bones mods for when those aren't available

df_caverns/primordial.lua

@@ -194,6 +194,7 @@ local jungle_cavern_floor = function(abs_cracks, humidity, vi, area, data, data_
 		local firefly_vi = vi + ystride * math.random(1, 5)
 		if data[firefly_vi] == c_air then
 			data[firefly_vi] = c_fireflies
+			minetest.get_node_timer(area:position(firefly_vi)):start(1)
 		end
 	end
 end
@@ -262,6 +263,7 @@ local jungle_warren_floor = function(abs_cracks, vi, area, data, data_param2)
 		local firefly_vi = vi + ystride * math.random(1, 5)
 		if data[firefly_vi] == c_air then
 			data[firefly_vi] = c_fireflies
+			minetest.get_node_timer(area:position(firefly_vi)):start(1)
 		end
 	end
 end

df_caverns/sunless_sea.lua

@@ -103,7 +103,9 @@ df_caverns.register_biome_check(function(pos, heat, humidity)
 	if pos.y < df_caverns.config.sunless_sea_min or pos.y >= df_caverns.config.level3_min then
 		return nil
 	end
-	if heat > hot_zone_boundary then
+	if pos.y < sea_level then
+		return "sunless undersea"
+	elseif heat > hot_zone_boundary then
 		return "barren" -- hot zone
 	elseif heat > middle_zone_boundary then
 		return "fungispore"

df_caverns/underworld.lua

@@ -6,6 +6,7 @@ local modpath = minetest.get_modpath(modname)
 local S = minetest.get_translator(modname)
 
 local bones_loot_path = minetest.get_modpath("bones_loot")
+local bones_node = df_dependencies.node_name_bones
 local named_waypoints_path = minetest.get_modpath("named_waypoints")
 local name_generator_path = minetest.get_modpath("name_generator")